Recommendation ITU-T G.709/Y.1331 defines the requirements for the optical transport network (OTN) interface signals of the optical transport network, in terms of: – OTN hierarchy – functionality of the overhead in support of multi-wavelength optical networks – frame structures – bit rates – formats for mapping client signals. Edition 6.4 of this Recommendation corrects the Cn(t) equations in clauses 19.4.3.2 and 20.4.3.2 and column number values 3807 and 3808 in Figures 20-1 and 20-2 into column number values 3808 and 3809.